Mobile telephony networks have experienced an enormous spread, especially after the introduction of Second Generation (2G) mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese counterparts). The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS), Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available to subscribers of the cellular network in the last few years.
In particular, a great interest is being devoted to multimedia services, that is, in addition to the voice-only person-to-person communication capability, the ability to add images, video, browser data, game board information, and so on.
U. Olsson and M. Nilsson, in the article “Combinational services—The pragmatic first step toward all-IP”, Ericsson Review No. 2, 2003, disclose, inter alia, an example of so called “combinational services”, in which the ability to simultaneously handle circuit-switched and packet-switched traffic is used: sharing pictures during conversation. The authors notice that the possibility of simultaneously handle circuit-switched and packet-switched is allowed both in WCDMA (Wideband Code Division Multiple Access), that gives the possibility to set-up multiple, parallel bearers over the air interface (multiple radio access bearers, multi-RAB), and in GSM, in which a standardized mechanism—dual transfer mode (DTM)—yields similar possibilities. With particular reference to the latter, the authors disclose that by means of careful slot allocation, DTM enables a single radio to have parallel, simultaneous circuit-switched (CS) and packet-switched (PS) capabilities, allowing a user to send MMS without interrupting an ongoing phone call.
Recently, another service has been proposed for subscribers of mobile networks: video-telephony. Generally speaking, video-telephony allows two users having respective mobile phones to see each other during a phone call. In practice, the mobile phones are provided with a video-camera, so that images recorded by the video-camera of the first user are sent, in real-time, synchronized with the voice, to the second user (and vice-versa), on the bearer supporting the call. This kind of communication requires high data transfer rates and high quality of service (QoS) level.
Video-telephony is currently serviced in UMTS (Universal Mobile Telephone System) networks, in which both video and voice data are carried along a circuit-switched bearer having high data rate, as permitted by UMTS. Video-telephony (or video-conferencing) is also available in fixed (PSTN) networks, in which a circuit-based (e.g. ISDN) connection is established for supporting the service.
PCT published patent application no. WO 01/03461 proposes a method for performing a communication between two terminal equipment via a communication network. The method comprises the steps of establishing a first connection for a communication signal between two terminal equipment, detecting a connection changing condition, and, if said connection changing condition is detected, dividing said communication signal at least into a first component and a second signal component, and establishing at least a second connection, the first signal component (for example a video component) being transmitted via the first connection and the second signal component (for example a speech component) being transmitted via the second connection. The second connection can be a connection similar to the first one (e.g. a UMTS multimedia call connection) or just a (lower capacity) speech call connection. Additionally, in case of a handover, in particular in a handover to a network with lower capacity or which is unable to perform a video-call (for example some GSM networks), by dropping the video component (i.e. the corresponding connection), the multimedia call or at least the speech call can be maintained. According to the authors, in general the second connection is only activated if required (low quality, preparation of handover or the like) and canceled when not needed anymore.
A recent technology developed in the mobile network field is the EDGE (Enhanced Data rates for GSM Evolution) technology, which has been introduced to boost network capacity and data rates of both circuit switched and packet switched domains in 2G mobile networks, to meet the demand of wireless multimedia applications and mass market deployment. EDGE uses an enhanced modulation, based on 8PSK (Phase Shift Keying), that increases the data rates by up to three fold. As a modification to existing GSM networks, EDGE does not require new network elements, and can co-exist with UMTS, for instance to provide high speed services for wide-area coverage, while UMTS is deployed in urban hot spots.
EDGE modulation increases the data throughput provided by a packet switched service even over 400 kbit/s per carrier. Similarly, the data rates of circuit switched data can be increased, or existing data rates can be achieved using fewer timeslots, saving capacity. Accordingly, these higher speed data services are referred to as EGPRS (Enhanced General Packet Radio Service), for packet-based services, and ECSD (Enhanced Circuit Switched Data), for circuit-based services. The data rates specified by ETSI (European Telecommunication Standard Institute) bring ECSD rates up to 38.4 kbit/timeslot and EGPRS rates up to 60 kbit/timeslot. For ECSD, it is possible to support a 64 kbit/s real time service with a low bit error ratio (BER) by allocating two time slots of 32 kbit/s each.
Document Tdoc SMG2 EDGE 033/99, titled “ECSD (Enhanced Circuit Switched Data)—concept evaluation v. 4.0”, presented by Nokia at the ETSI SMG2 Working Session on EDGE, 2-4 Mar. 1999 (Toulouse, France), presents a concept evaluation for ECSD services. According to the document, one of the main applications for ECSD are the video applications. Besides data rates there are other requirements for video transmission. Video services usually include several components, each of them with varying QoS requirements. FIG. 1, extracted from the above mentioned document, shows two concepts in terms of QoS of providing video-telephony service (e.g. video-conferencing call). The differences between the two concepts are in the way of ensuring the QoS for different components: audio, video and data. The concept shown in the FIG. 1 (left) has no means of differentiating QoS for different components. Therefore the QoS of the call should match the QoS requirements of the most demanding component. The concept in FIG. 1 (right) offers possibilities of QoS differentiation for different service components of the call, making the utilization of resources more flexible. However, as observed in the document, in circuit ECSD there is no possibility for providing QoS differentiation without multiple connections.